The doc node represents the top node of the RSS file, which
in RSS 0.9 was identified by <rdf:RDF xmlns="..." rdf:xmlns="...">.
This established RSS as an application of the Resource Description
Framework (RDF), using namespaces to distinguish RDF tags from
RSS elements. RSS 0.91 declared <rss version="0.91">
as the root node, so my code fix now tries to find one or the other,
and gives up with an error message if it could not.